1. Field of the Invention
The present invention relates to a semiconductor laser apparatus used in an optical disk apparatus, a laser printer, an optical data communication system, and the like.
2. Description of the Related Art
Semiconductor lasers are widely used in systems of optical disk apparatuses, which are large capacity memory apparatuses, or laser printers, because of advantages of easy modulation of direct light intensity, a small size, low power consumption and high efficiency.
Conventional semiconductor lasers, however, have a disadvantage in that the amount of emission light varies due to the following characteristics:
(1) A variation in differential quantitative due to a temperature variation and a variation with the passing of time,
(2) A variation in threshold current due to a temperature variation and reflection light (return light), and
(3) Occurrence of mode hopping noise due to reflection light (return light).
In order to drive the semiconductor laser, a control circuit for monitoring and stabilizing the amount of output light of the semiconductor laser is indispensable. In particular, in an optical disk apparatus, in order to increase the memory capacity and data transfer rate, light intensity modulation with higher precision and reduction in noise at the time of reproduction are required.
A wide band front APC method is known as a method for reducing laser noise with a currency available semiconductor laser used as a light source of an optical disk apparatus (e.g., TAGUCĤI, HOSHINO: xe2x80x9cHigh-Precision Laser Control System (11)xe2x80x9d in Optical Disk Apparatuses, General Meeting in Spring 1991 of the institute of Electronics, Information and Communications Engineering, C-372, etc.).
In the wide band front APC method, a light beam actually radiated on an optical disk in a recording/reproducing mode, i.e., part of a front beam of a semiconductor laser is guided to, and detected by, a photodetector. A detection signal from the photodetector is used for light output control of the semiconductor laser. According to this method, the control band is increased and thus the laser noise is reduced.
Regarding the wide band front APC, it is important how the control band is increased in relation to the reproduction signal band. A technique for making the control band of the wide band front APC wider than the reproduction signal band is disclosed in, e.g., Jpn. Pat. Appln. KOKAI No. 4-209581 (the title of the invention: xe2x80x9cSemiconductor Laser Apparatusxe2x80x9d).
The semiconductor laser apparatus is provided with an error detection circuit for outputting an error signal representing an error between an output signal negatively fed bank from a photodetector for detecting an output beam of the semiconductor laser and an external control signal. A feedback loop is formed which controls a drive current for the semiconductor laser on the basis of the error signal. In addition, the semiconductor laser apparatus is provided with a compensation circuit for negatively feeding back a compensation current for compensating a phase delay of the feedback loop to an input terminal of the error detection circuit.
In the prior art, however, no consideration has been paid to the variation in feedback amount of the feedback loop due to the aforementioned variation in differential quantitative efficiency or the variation with the passing of time of the semiconductor laser, or the variance in adjustment of optical systems among apparatuses.
Nor has consideration been paid to the variation in frequency characteristics due to a delay in the semiconductor laser or a variation in junction capacitance in the photodetector.
If the aforementioned variation feedback amount or frequency characteristics occurs, laser noise cannot fully be reduced, in particular, in an information recording/reproducing apparatus such as an optical disk apparatus. Furthermore, unnecessary noise occurs due to degradation in transient response characteristics to noise.
Moreover, the degradation in transient pulse response characteristics at the time of recording poses a more serious problem, since it result in a recording mark variation and greatly loses a reproduction margin.
On the other hand, in a laser printer, etc., there is a demand for a much higher light turn/off ratio, i.e., a light turn on/off ratio. However, in a turn-off region, i.e., a threshold lower than laser oscillation, the feedback efficiency is greatly lowered and high light turn-off ratio cannot be obtained. This problem will now be described with reference to simulation results shown in FIGS. 1 and 2.
FIG. 1 shows a response waveform of a monitor PD current at the time of turn on/off. It is understood that the control system is deteriorated because the light is not fully turned off at the time instant of turn off, although the light has high-speed responsiveness and stable at the time of turn on.
FIG. 2 shows a response waveform of an LD (laser diode) driving current at the time of turn on/off. It is understood that the driving current can be controlled to only the level of threshold current at the time instant of turn off.
As has been mentioned above, in the conventional semiconductor laser apparatus, no consideration is paid to the variation in feedback amount of the feedback loop due to the variation in differential quantitative efficiency or the variation with the passing of time of the semiconductor laser, or the variance in frequency characteristics due to a delay in the semiconductor laser or a variation in junction capacitance in the photodetector.
Thus, in the case of the information recording/reproducing apparatus such as an optical disk apparatus, the effect of laser noise reduction is not sufficiently and unnecessary noise is caused by the degradation in transient response characteristics to noise.
In the case where the performance in light turn-off ratio is required, the feedback efficiency decreases greatly at a level lower than the laser oscillation threshold and a high light turn-off ratio cannot be obtained.
An object of the present invention is to provide a high-powered semiconductor laser apparatus wherein the feedback amount of a feedback system for negatively feeding back a driving current of a semiconductor laser is automatically compensated without damaging the dynamic range of a controllable amplifier, the variation in characteristics due to a delay in a semiconductor laser or a variation in junction capacitance in the photodetector, and a high turn-off ratio is obtained.
To attain the above object, there is provided a semiconductor laser apparatus comprising:
a semiconductor laser;
a photodetector for detecting an output beam of the semiconductor laser;
a driving unit having a plurality of driving elements connected in parallel, for driving the semiconductor laser;
a controllable amplifying unit for receiving an externally supplied control signal and a monitor signal of the photodetector and supplying a drive signal determined by the control signal and the monitor signal of the photodetector to the driving unit, thereby feedback-controlling the output beam of the semiconductor laser; and
a compensation unit for actively controlling a phase of a feedback control loop constituted by the photodetector, the driving unit and the controllable amplifying unit in accordance with temporal and electrical behaviors of the feedback control loop.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.